1. Field of Invention
The present invention relates to the field of marine seismic instruments and methods of controlling same. More specifically, the invention relates to apparatus and methods for remotely controlling the position of marine seismic instrumentation, as well as related systems, methods, and devices.
2. Related Art
Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. For large survey areas, a single vessel may tow one or more seismic sources and one or more seismic streamer cables through the water. Alternatively, a tow vessel may either be a “pure” source vessel (meaning it only tows seismic sources) or a “pure” streamer (receiver) vessel, in which case two or more vessels may be used. In any case the seismic sources may comprise compressed air guns or other means for generating acoustic pulses in the water. The energy from these pulses propagates downwardly into the geological formations and is reflected upwardly from the interfaces between subsurface geological formations. The reflected energy is sensed with hydrophones attached to the seismic streamers, and data representing such energy is recorded and processed to provide information about the underlying geological features.
Three-dimensional (3-D) seismic surveys of a grid provide more information regarding the subsurface formations than two-dimensional seismic surveys. 3-D surveys may be conducted with up to twelve or more streamers that form an array covering a large area behind the vessel. The streamers typically vary in length between three and twelve kilometers. Tail buoys attached at the streamer distal ends carry radar reflectors, navigation equipment, and acoustic transponders. Hydrophones are positioned along each streamer. The in-line interval between each receiver, or receiver group, ranges between about 3 and 25 meters, with 12.5 meters comprising typical interval spacing.
Since the grid is often much wider than the array, the tow vessel must turn around and tow the array in laps across the grid, being careful not to overlap or leave large gaps between the laps across the grid.
A multiple streamer array requires deflectors near the vessel to pull the streamers outwardly from the direct path behind the seismic tow vessel and to maintain the transverse or crossline spacing between individual streamers. The same is true for multiple sources being towed behind a tow vessel when no streamers are present. Deflectors rely on hydrodynamic lift created by forward motion through the water to pull the streamers and/or sources outwardly and to maintain the transverse position relative to the vessel path.
In 4-D geophysical imaging, a 3-D seismic survey is repeated over a grid that has been previously surveyed. This series of surveys taken at different times may show changes to the geophysical image over time caused, for example, by extraction of oil and gas from a deposit.
It is important that the source members being used to generate the acoustical pulses be located as closely as possible to the same location as in previous surveys over the same grid. This has been difficult to accomplish in a marine survey because the acoustical source members are typically towed behind the tow vessel in source arrays, which are subject to wave and current movement.
In addition to the deployment and operation difficulties associated with towing multiple streamers and/or multiple source arrays, conventional techniques limit the ability to position source arrays and streamers in different relative positions and orientations. Source array design is limited by the tow configuration. Each towed source array is also subject to crosscurrents, wind, waves, shallow water, and navigation obstacles that limit the coverage provided by the survey system.
Attempts to control the location of seismic sources and source arrays have included attaching distance ropes running to lateral passive deflectors and tow cables; use of active (steerable) deflecting members attached to the source tow cables in front of the source arrays, or mid-way or at the aft end of the source arrays; and use of passive lateral deflectors equipped with a winch located near the front of the source. WO2004092771 A2, published Oct. 28, 2004, (the '771 application) discloses the latter two options. By attaching one or more steerable deflecting members to the front, rear, or mid-section of one or more source arrays, or a winch to the front of the source that acts on a passive lateral deflector, the source array locations may be controlled. Another method and device employs a source array comprising a rigid bar mounted under a rigid or semi-rigid float member, with the seismic source members, for example air-guns, hanging below the rigid bar. FIGS. 1A and 1B illustrate plan and side-elevation views, respectively, of this source array 100. Source array 100 comprises a rigid steel or aluminum member 8 rigidly mounted to a rigid or semi-rigid float 10, which floats near surface 12 of the ocean or other water body. Multiple source members 14 are hung by chains or other means 16 from member 8, and source array 100 is towed behind a seismic vessel (not shown) by a strength-taking source umbilical 2 that is attached to a tow bridle having two elements, a front element 4 attached to a front 5 of member 8, while a second bridle element 6 is attached proximate a mid-section 7 of member 8. The lengths of bridle elements 4 and 6 determines the orientation, or so-called angle of attack of member 8 and float 10 toward the incoming flow, F. Therefore, member 8 and float 10 function as a low aspect ratio hydrofoil creating lateral lift that enables source array 100 to be laterally deflected. However, this method and apparatus offers no possibilities for remotely adjusting the angle of attack.
The previous attempts have not provided optimal control of the location of the source arrays under towing conditions. While these techniques are improvements in the art, further improvement is desired.